Field of the Invention
The present invention relates to the field of microbiology and food technology. In particular, it relates to a method for improving the quality of soy sauce by use of a new Bacillus amyloliquefaciens strain.
Description of the Related Art
Aroma and safety are major factors in evaluating soy sauce quality. Currently, methods for enhancing soy sauce aroma has been a hot topic. Higher content of flavor components was realized by adding Tetragenococcus halophilus and salt-tolerant yeasts during traditional production process of Chinese soy sauce, while the content of some key indicators of soy sauce including total nitrogen, amino nitrogen, and reducing sugar has not been changed. The amount of total amino acids and flavor components in fish sauce were increased using Tetragenococcus halophilus as an initial strain in soy sauce fermentation. However, methods for both enhancing aroma and reducing the content of ethyl carbamate, a reported cancinogen, in soy sauce has so far not been reported.
In recent years, there are increasing attention on food safety, particularly on traditional food and condiment. Ethyl carbamate (EC) has been widely detected in fermentation products. It has been classified as class 2A carcinogen by International Agency for Research on Cancer (IARC) for its potential to induce lung cancer, liver cancer, skin cancer and other kinds of cancers in human. EC has also been widely detected in fermented foods such as wine, rice wine, soy sauce and soybean paste. Researches on the process for producing soy sauce revealed that EC was formed by ethanol (the ethanol content is 2.5% in Japanese soy sauce) reacting with the main precursors, citrulline and urea.
Currently, there is rarely any research on how to control or reduce EC in soy sauce. Some researchers attempted to decrease EC content by reducing the amount of EC precursor, citrulline, by adding T. halophilus during soy sauce production. However, it is uncertain whether the final concentration of EC in soy sauce could be lowered or not by adding T. halophilus, and its effect on aroma or quality of soy sauce also remains unknown. Other researches on EC reduction were mainly focused on alcohol beverages, which were carried out by adding acidic urease, constructing engineered Saccharomyces cerevisiae with low urea production, or adjusting production process.
The precursors to make EC in the soy sauce production are citrulline and ethanol. Decreasing the amount of citrulline in the soy sauce production is therefore an effective way to reduce the EC content in the final product.
In the preproduction of soy sauce, large amount of arginine is generated by hydrolyzing raw materials. The arginine is converted into citrulline by a large class of anaerobic or facultative anaerobic lactic acid bacteria. If the arginine can be used by bacterial strains that have a complete arginine de-iminase pathway that can convert arginine to ornithine, the accumulation of citrulline in soy sauce can be reduced.
The goal of present invention is to provide a new salt-resistant Bacillus amyloliquefaciens strain that can effectively convert arginine to ornithine without accumulation of citrulline under high salt conditions and can be used to reduce the EC content in soy sauce. In addition, it is found that the new strain not only can reduce the EC content, but also can enhance the aroma and taste of the soy sauce product.